It is often desirable to provide amplification of signals, such as radio frequency (RF) signals, for further signal processing. Additionally, it is often desirable to provide such signal amplification with variable gain. For example, with respect to some signals the objective may be to create a constant output signal with a varying input signal. Such a situation is typical in a receiver where, because of signal fading, changes in the transmission medium, changes at a corresponding transmitter, etcetera, a signal or signals having time varying amplitudes may be provided to a receiver input, but a constant signal level is desired at the output, such as to provide a constant sound level or to facilitate further signal processing. Accordingly, a variable gain amplifier (VGA) may be utilized in the receiver to adjust the gain as a function of the received signal level in order to equalize the output level.
Alternatively, with respect to other signals the objective may be to create a variable output signal with a constant input signal. Such a situation is not uncommon in a transmitter where, depending upon the final application for the transmitter, the particular transmission medium utilized, changes in the transmission medium, changes at a corresponding receiver, etcetera, a signal or signals having constant amplitudes may be provided to a transmitter input, but a selectable or variable signal level is desired at the output, such as to provide a desired signal level at a receiver irrespective of a transmission medium.
One situation in which a variable gain amplifier may be used in transmission of signals is in a cable television head-end system. In such a situation the signal or signals to be transmitted are often provided to the transmitter with a relatively constant amplitude. Due to the particular application a transmitter is deployed in, changes in the signal transmission path, etcetera, the signal output level of the transmitter may be varied, such as to provide a desired or relatively constant signal level at downstream cable equipment. However, such an environment requires highly linear signal processing so as to introduce very little distortion into the signals. Specifically, a cable transmission system typically transmits a large number of broadband signals which are closely spaced, resulting in a very dense signal environment. Accordingly, high linearity is required in order to avoid not only distortion of the individual signals, but to avoid interference among and between the signals.
Therefore, a need exists in the art for providing very linear amplification of signals. Moreover, a need exists in the art for highly linear amplifiers to provide variable gain. A further need exists in the art for a highly linear variable gain amplifier, such as may be utilized with respect to the transmission of cable television signals.